Ultra-wideband (UWB), a wireless transmission technology for high bandwidth (480–1320 Mb/sec) and short range (10–50 m), is gradually seeing more use in medical applications. It was initially developed as technology for the military—only after the U.S. military lifted the secrecy in 1994 did development for commercial purposes begin. Early UWB chip sets were geared toward replacing the USB cables in mainstream PCs. However, the requirements for medical applications are different because transmitting real-time video and ultrasound images requires low latency and deterministic data throughput. Another hurdle in using UWB technology is that vendors of commercial UWB chip sets require volumes of several hundred thousand orders per year. However, there are now companies that offer UWB especially geared toward the needs and the volumes of the medical market. Manufacturers of medical instruments have begun using it for video endoscopes, laryngoscopes, and ultrasound transducers. This article describes how UWB can be harnessed for video endoscopes.

Endoscope Considerations

Flexible optical endoscopes have a long, thin tube that is introduced into the body of a patient. New endoscopes contain the light source and a tiny image sensor directly at the tip. This construction is possible with the availability of new LED light sources and miniature CMOS cameras. An LED light source directly at the tip of the endoscope consumes much less power than conventional high-power light sources. As a result, a small battery pack is sufficient to operate the instrument for a couple of hours. In addition, the expensive optical pipes can be replaced with copper wires. A further advantage is that the image can be displayed on a LCD monitor and recorded at the same time. The wireless link to the monitor eliminates the endoscope’s physical constraints, which can make procedures more comfortable for both the patient and the physician.

Digital transmissions are ideal because they offer high picture quality and avoidance of distortions. Because the physician is watching his manipulations on the patient on a video monitor, images should appear on the screen in real time—in other words, with as little latency as possible. Therefore, the video signal must not pass through compression circuits or extensive protocol stacks. UWB’s high bandwidth, low latency, low radiation, and robustness make it an appropriate wireless transmission technology for this instrument.

UWB Radio Technology

Transmitting an uncompressed video in NTSC quality requires a deterministic data rate of at least 166 Mb/sec, a rate that conventional technologies don’t fulfill. Conventional wireless technologies use a radio access mechanism dependent on the availability of the channel. This means that other devices within reception may temporarily reduce the data bandwidth. With UWB technology, a channel can be permanently reserved during a session. UWB technology offers low protocol overhead, which is important for achieving a short transmission latency time. A robust radio link can be achieved by spreading the data over 128 subcarriers. Further advantages and details are described in the following sections.

 

UWB Radio Layer

Earlier developments of UWB were based on different physical (PHY) and medium access control (MAC) layer specifications. In the last three years, the MAC and PHY layer specification of the WiMedia-Alliance has been adopted by most UWB implementers. Unlike the established wireless transmission technologies (e.g., WLAN), UWB uses a band of 528 MHz per transmission channel. For comparison, a WLAN channel uses a maximal bandwidth of 20 MHz. Three 528-MHz bands build one band group. The total frequency range available for UWB spans from 3.1 to 10.6 GHz and is divided into five band groups. Advanced dual-band transceivers are available for band groups one and three.

Orthogonal frequency division multiplexing modulation is the modulation technique for WiMedia-UWB. Each 528-MHz band is divided into 128 subcarriers, where each subcarrier has its peak at the zero point of its neighbor (hence the name orthogonal; see Figure 1, p. 27). Transmit information is divided amongst the 128 subcarriers with each 528-MHz channel carrying a maximum of 480 Mb/sec.

The spreading of subcarriers over a large 528-MHz band allows for a very low transmit power of 37 μW (WLAN, in comparison, is allowed to radiate more than 300 mW). Both the availability of a wide band for transmitting the information and a very low transmit power result in a very high coexistence within the radio-frequency (RF) world. Although transmit power is only 37 μW, the transmission reaches a distance of up to 10 m and passes through a 25-cm brick wall without trouble.

 

Media Access Control Layer

While the UWB radio layer takes care of the RF processing, the media access control layer is responsible for managing the UWB network and controlling the radio state. When several UWB devices are in the same vicinity, they hook up to a so-called ad hoc network. This is a not planned network. Rather, it is built by participants in the vicinity, and the members may join and disappear as they see fit.

Figure 2 shows three UWB devices building an ad hoc network, where device A is out of sight from device C. It is possible that device A, which is located to the left, knows of the existence and occupied slots of device C, even if it can’t hear device C. Device A knows about device C through so-called beacons. Devices learn from each other since the beacons contain information about their neighbors. Direct data transmission in every direction is possible between all devices that can receive each other.

UWB uses time division multiple access (TDMA), which means that transmission is organized in time slots and frames. UWB transmission slots are combined to superframes (see Figure 3). A superframe is divided into beacon period (BP) and data transfer period (DTP). Beacons and payload occupy 256 medium access slots of the superframe. One medium access slot lasts 256 microseconds, and one superframe lasts 65.5 milliseconds. All network members that hear each other synchronize to the superframe by listening to received beacons. The information inside the beacons can be considered as the network members’ communications channels.

Due to the organization in time slots, not every device needs to receive and transmit data all the time. A device can wake up every 65.5 milliseconds to listen to beacons and if it doesn’t have any tasks, it can go back to sleep, similar to a cell phone. This results in long life on battery-powered systems.

Wireless interfaces are like cables: If there are several members but only a limited amount of communication channels, the access rights must be managed. A member that intends to send information to a channel listens to determine whether the channel is already occupied. If it finds that the channel is free, the member sends its message.

Two devices can listen to the channel at the same time, find out that the channel is free, and send their messages at the same time. This is called a collision. In this situation, the devices try to access the channel again later, and each device waits a random period of time before it accesses the channel again. The device with the higher priority may access the channel sooner than the device with lower priority. This contention access method was invented in the 1970s with Ethernet and is also typical for WLAN. However, this method can be contrary to the objective of transmitting a stream of video without interruptions and with minimum latency.

To guarantee an uninterrupted video flow, UWB uses the distributed reservation protocol (DRP). Because UWB is based on TDMA, a network member can reserve fixed time slots (media access slots) for communication with another device. Information about which time slots are occupied is transmitted during the beacon period. If a time slot is marked as a hard reservation, no third party can occupy it. This method is necessary to accomplish deterministic data rates, e.g., for video transmission. Besides DRP access, UWB can also use prioritized contention access.

 

Implementation

Figure 4 shows the block diagram of an endoscope camera unit. The block diagram for the monitor is similar, except that the digital video interface is replaced by a display controller. The UWB-PHY is based on the RTU7012 Dual Band PHY by Wionics Research, complying with the WiMedia PHY 1.1 and PHY 1.2 specifications. It can operate in band groups one and three.

In this example, the UWB streaming MAC was designed by Zurich University of Applied Sciences to be implemented in ASIC or FPGA and was optimized for achieving a low-latency data transmission. To easily integrate the MAC into any system-on-chip, the ARM advanced host bus (AHB) is used as the data transfer bus and the ARM peripheral bus as the control bus. These interfaces make it especially suitable for integration into ARM-based system-on-chips.

Many parameters of the UWB standard are controlled by the microcontroller firmware, which requires no hardware changes if other higher-level protocols need to be added, e.g., wireless USB. Implementation in firmware also lowers the risk and adds flexibility in case of specification changes.

The MAC may transfer data between UWB devices in any direction and is not restricted to video. For this particular video application, signals from the camera are transferred via the digital video interface and AHB to the SDRAM, which acts as an intermediate video buffer (see Figure 5). The MAC picks up video data from the SDRAM and transfers them to the UWB network for transmission. Conversely, data received by the UWB PHY are transferred to the SDRAM.

The MAC acts as AHB bus master when transferring data between the UWB network and SDRAM without intervention of the processor core. The processor, now free of data transfer tasks, can instead be used for controlling the MAC setup of subsequent UWB superframes. Such architecture allows any AHB bus device to be used as a target or source for data transfers to and from the UWB-MAC. For interfacing to the UWB radio module, the UWB-MAC uses the WiMedia ECMA369 MAC-PHY interface standard.

Other necessary components of the endoscope are AD convertors and pulse-width modulators used for battery management. A standard cell ASIC is a suitable choice to fit everything into the handle of the endoscope and to keep power consumption low. However, since the predicted volumes do not justify the development costs of a standard cell ASIC for this example, a customizable application processor (CAP) was used. This ARM-based microcontroller possesses all the commonly required peripherals and software drivers plus an additional metal programmable logic area for customer-defined functionality. The UWB-MAC and the digital video interface are implemented in the metal programmable area of the CAP, which is similar to a gate array. Other standard peripherals of this microcontroller, such as the external bus interface, are used to control SDRAM without the technical risks and costs involved with a memory controller design.

To facilitate UWB application development, some vendors provide a CAP UWB evaluation kit. The fixed portion of the CAP device is implemented as a microcontroller standard product, coupled to a high-density FPGA that emulates the metal programmable block. The kit can be rapidly configured to emulate the behavior of a design under development. The UWB-MAC is implemented along with any application-specific logic in the FPGA. The UWB-PHY is implemented in an extension board. The CAP UWB evaluation kit interfaces with a PC running industry-standard ARM development tools for system development and debug. This configuration permits parallel hardware and software development that significantly reduces the development time for the application. When the system has been fully debugged, the UWB-MAC and application-specific logic are remapped into the MP block of a customized CAP, providing a complete UWB transceiver based on a small part count.

 

Conclusion

This low-cost, medium-volume UWB device can be used for wireless medical applications. It is suitable for unidirectional video links and could also replace the thick cables of ultrasound transducers, while providing the necessary galvanic patient isolation. Dental x-ray film is currently being replaced by x-ray scanners, which are placed in the mouths of patients. UWB can be used to link to image displays and storage devices. In addition, positioning peripherals of surgical robots may exchange data via a reliable UWB link.

UWB is an emerging technology that offers low latency and deterministic high data rate transfers with low power consumption and low electromagnetic radiation. Medical device manufacturers have begun using the technology for video endoscopes and ultrasound transducers, in part because UWB offers a deterministic data rate that conventional technologies do not fulfill. In addition, UWB offers low protocol overhead, which is important for physicians to be able to see movement with low latency or in real time.

 

Hans Gelke is a lecturer and researcher at Zurich University of Applied Sciences (Winterthur, Switzerland). He can be reached at hans.gelke@zhaw.ch.